Motoring into the future

Electric motors consume more than half the world’s electricity. Scientists are working hard to make them lighter and more efficient – to where they might eventually power commercial aircraft.

Electric motors are, without a doubt, a mature technology—more than a century separates Nikola Tesla’s invention of the induction motor and the launch of the Tesla electric car.

Even so, there’s still room to make them more energy efficient, to cut their cost of production, boost their resilience, reduce the amount of waste heat they generate and trim their weight.

That’s crucial. Not only do electric motors consume half the world’s electricity in powering much of modern life—from fans, compressors, pumps and appliances to the engines of cars and trucks—but better efficiency would also open up exciting new possibilities, such as electrically powered commercial flight.

Rare benefits

The interaction of magnetic forces is what makes electric motors work. These forces come from a combination of permanent magnets and fluctuating fields typically created by the flow of electricity through coiled copper wire. This can generate considerable waste heat that can affect the motor’s performance. So getting more out of permanent magnets is one way scientists are pushing electric motor technology along.

The most powerful permanent magnets are made from rare earth elements, which, as their name suggests, are expensive. Jun Cui, a senior scientist at the US Department of Energy’s Ames Laboratory, is looking for lower-cost alternatives. Candidates include inexpensive manganese alloys, which have the added benefit of being light, and ferrites. Finding the right replacement, however, means overcoming a couple of major hurdles.

Design duo

“Permanent magnets have two engineering issues,” says Cui – including the newer candidates. One is temperature dependence, and the other is brittleness.

Machines that rely on magnets are, of course, engineered to operate below the demagnetising threshold, but if a motor runs hot it can become completely demagnetised, he says.

The traditional fix is to incorporate the rare earth metal dysprosium, because it can stabilise a magnet at high temperatures. But, as well as being expensive, dysprosium is almost solely supplied by China. That monopoly is a real problem, Cui says.

Another problem is that the most powerful permanent magnets are also the hardest, which means they’re also the most brittle. Indeed, so much so that up to 30 per cent of them break during manufacture.

“Your machining rate is roughly 70 per cent,” Cui says. “We’re trying to solve this brittleness problem.”

Cui’s lab is exploring various approaches to limiting the ability of cracks to propagate in a magnet’s microstructure including adding fine fibers and carbon nanotubes that work like the steel in reinforced concrete.

Fans for flying

At the University of Arkansas, Fang Luo, an assistant professor of engineering, has grand plans for advanced electric motor drives. He would like for them to replace the fossil-fuel-operated components of really large engines. For example, jet engines, which Luo describes as “just basically high-speed, powerful fans,” could eventually be powered by electricity. Electrifying propulsion systems will reduce transportation costs and cut carbon-dioxide emissions, Luo adds.

One crucial component that will need to be improved is the motor drive — the electronic device that controls the speed of the motor —which is Luo’s area of expertise. The more powerful the engine, the more heat the drive generates.

“You don’t want to cook your passengers,” says Luo. “You need a lot of space and weight to manage that heat. These extra thermal-management parts will shade the overall benefit of replacing fossil-fuel-operated engines.”

To overcome this requires pushing up the efficiency, and driving down the weight as much as possible.

One solution Luo is experimenting with is what he calls “magic” silicon carbide, or SiC: “a new material that gives way better performance than the silicon devices we were using for so many years.”

The SiC electronics lose only one-tenth of the power that conventional silicon does in an electric motor, meaning that a SiC-powered electronics motor drive can be much smaller than current alternatives. Luo is working with colleagues from the U.S. National Science Foundation-supported Center for Power Optimization for Electro-Thermal Systems on a prototype motor drive based on his technology.

“If you really want to shrink the size of motor-drive systems, it’s no longer just an electrical engineering thing,” says Luo. “It’s an interdisciplinary effort.”